The present invention relates to the formation of emulsions and dispersions, and in particular, to the process of formation of stable oil emulsions and particulate dispersions of hydrophobic materials in aqueous substances.
Everyday “common knowledge” tells us that oil and water do not mix. This fact is assumed to be related to the different properties of the two materials. Oil being nonpolar is “hydrophobic” and therefore water-hating. This view is certainly valid for molecular mixtures of oil and water. Simply put, the work required to force apart hydrogen-bonded water molecules to incorporate hydrocarbon solute molecules is too large and this work is not compensated by a strong solute-water bonding, which is the case for soluble solutes such as common salt, sugar and ethanol. Each of the latter solutes can form strong bonds with adjacent water molecules, which replace those between the water molecules themselves.
Another type of mixture, not molecular in nature, can be formed by the dispersion of microscopic droplets or particles, often in water, to form a colloidal solution or dispersion. However, hydrocarbon oils and finely divided hydrophobic particles will not readily disperse in water and will only remain stable for a short length of time, typically, for less than an hour, even after vigorous mechanical agitation. Thus, most industrial processes involving these mixtures require continuous agitation and fairly rapid reaction times. The efficiency of these processes is also reduced by the difficulty in maintaining small particle sizes and a high reaction surface area, because of the tendency to coalescence and coagulation. The stability of emulsions and dispersions can be much improved by the addition of surfactants and polymers, which can change the nature of the oil/water interface. However, these observations are, at first sight, not consistent with a simple application of the well-established DLVO theory of colloid stability (J. N. Israelachvili, Intermolecular and Surface Forces, Academic Press (1992)). This is because oil droplets and fine, hydrophobic particles, even without additives, are known to develop significant surface electrostatic potentials in water and, in addition, generally have weak van der Waals attractive forces. These conditions usually indicate that the colloidal dispersion will be stable.
It is, therefore, of some interest to consider in more detail why hydrophobic colloids, such as fine oil droplets, fail to maintain colloidal stability in water. The molecular force or bonding argument is not valid in this case (although it is directly related to the ultimate thermodynamic stability of the colloidal solution) because colloidal dispersions can be maintained in a meta-stable state by the operation of strong repulsion forces generated between the particles or droplets. The DLVO theory is based on the assumption that it is the charge on a colloidal particle (or droplet) surface which generates a repulsive electrostatic (double-layer) force between particles, which can be of sufficient strength to overcome the ubiquitous van der Waals attractive force. These combined forces can be estimated from the theory using the measured electrostatic potential on the surface of, for example, typical hydrocarbon oil droplets and their van der Waals attraction. The surface potential of dodecane droplets (as well as some other oils) was carefully measured in 1996, (Marinova, K. G. et al., Langmuir 12(8):2045-2051 (1996)), and these values can be used to calculate the expected interaction energy between two oil droplets in water. In order to consider the static forces involved, reference is made to FIG. 1. FIG. 1 gives the estimated DLVO interaction energies expected for 0.3 μm radius dodecane oil droplets in water, using the simplified, i.e., linear (low potential), Poisson-Boltzmann equation, combined with a nonretarded Hamaker attraction and using the Derjaguin approximation (J. N. Israelachvili, Intermolecular and Surface Forces, Academic Press (1992)). This radius was selected because smaller droplets will tend to coalesce through kinetic collisions and larger droplets have a greater buoyancy force and will rapidly coalesce with the upper oil phase. Droplets of 0.3 μm radius will rise at the rate of nearly 0.5 cm per day in water.
The DLVO equation used here to calculate this interaction is given below:
            V      s        ⁡          (              /        kT            )        =            a      kT        ⁡          [                        2          ⁢                                          ⁢          π                ⁢                  ∈          0                ⁢                              D            ⁢                                                  ⁢                          ψ              0              2                        ⁢                          exp              ⁡                              (                                                      -                    κ                                    ⁢                                                                          ⁢                  H                                )                                              -                                    A              121                                      12              ⁢              H                                          ]      
where Vs is the interaction energy between spheres in kT units, “a” is the spherical radius, ψ0 is the particle's electrostatic potential, κ−1 is the Debye length, A121 is the Hamaker constant for the system, and H is the interparticle separation distance. ε0D is the permittivity of water. In the dodecane/water system, the calculated barrier is almost 800 kT (see FIG. 1), which would give long-term stability to emulsion droplets of this size (Hiemenz, P. C. et al., Principles of Colloid and Surface Chemistry: New York (1977)). The significant surface potential reported (Marinova, K. G. et al., Langmuir 12(8):2045-2051 (1996)), of between −50 and −60 mV, is apparently caused by the specific adsorption of hydroxyl ions from the aqueous phase to the oil/water surface. The standard theory of coagulation of colloids (Hiemenz, P. C. et al., Principles of Colloid and Surface Chemistry: New York (1977)) indicates that coalescence or coagulation will become significant when the repulsive barrier approaches 10-20 kT or less. Clearly, these simple calculations indicate that fine droplets of hydrocarbon should be indefinitely stable against coalescence in water, because of a combination of surface charging via hydroxyl ion adsorption and a relatively weak van der Waals attraction, a result which is at complete variance with general observation.
Because the interparticle potential energy barrier is proportional to the droplet radius, smaller hydrocarbon oil droplets will readily coalesce, as observed for the fine droplets (less than 50 nm radius) produced by the sonication of oil and water mixtures (Sakai, T. et al., Langmuir, 17(2):255-259 (2001)). Larger droplets will feel a substantial buoyancy force and will readily combine with the upper phase of dodecane liquid, because it has a lower density than water. Droplets of intermediate (micron) sizes should be stable for longer periods, if only DLVO forces operate. Similar stability arguments can be applied to fine Teflon particles, which are known to be charged (Kratohvil, S. et al., J. Colloid Interface Sci., 57(1):104-114 (1976)) and have an even weaker van der Waals attraction in water (with a Hamaker constant of 0.3×10−20 J) (J. N. Israelachvili, Intermolecular and Surface Forces, Academic Press (1992)). The failure of the DLVO theory to predict the behavior of oil in water emulsions raises the issue of whether there might be additional forces involved. About 20 years ago, a new long-range attractive force, called the “hydrophobic interaction” (Israelachvili, J. N. et al., Nature, 300:341-342 (1982)), was discovered which acted over relatively large distances (>10 nm) between hydrophobic surfaces in water. Subsequent workers have extended the range to over 100 nm in some systems.
In later work, it was also suggested that dissolved gases may play a significant role in this interaction (Craig, V. S. J. et al, J. Phys. Chem., 97:10192-10197 (1993); Karaman, M. E. et al., J Phys. Chem. 100:15503-15507 (1996)), which has yet to be properly described by any theoretical model. At normal temperature and pressure, dissolved air in water has a concentration of around 1 mM, corresponding to about 20 mL of dissolved gas in a liter of water. In typical alkanes, gas solubility can be an order of magnitude higher. The role of dissolved gas in the properties of solutions has hardly been considered theoretically and, experimentally, only sporadically and often ignored.
In 1993, it was demonstrated that partial degassing, using a water jet pump aspirator to produce about 97% degassing for mixtures of dodecane/water, increased the stability of oil droplets on vigorous shaking (Craig, V. S. J. et al, J. Phys. Chem., 97:10192-10197 (1993)). These initial observations stimulated an AFM study which attempted to address the issue of dissolved gas in the interaction of hydrophobic polypropylene surfaces (Meagher, L. et al., Langmuir 10(8):2736-2742 (1994)). The polypropylene surfaces used in that study, although very hydrophobic as measured by the water contact angle, generated only a relatively short-ranged attraction measurable up to about 25 nm separations. The effect of degassing was only minor in this case. However, Horn et al. (Considine, R. F., et al., Langmuir 15(5):1657-1659 (1999)) reported a significant decrease in the long-range attraction (20-400 nm) between two polystyrene latexes as the level of dissolved gas in solution was reduced. In further work on emulsion stability, using a water jet pump to degas to the 97% level, it was also demonstrated that dissolved gas plays a significant role in emulsion stability and emulsion polymerization (Karaman, M. E. et al., J. Phys. Chem. 100:15503-15507 (1996)) and this led to the suggestion that dissolved gas may play a significant role in the balance of hydrophobic and hydrodynamic (drainage) forces responsible for the phase separation. By comparison, Zana et al. examined the effects of degassing on surfactant aggregation (Alargova, R. G. et al., Langmuir 14(7):1575-1579 (1998)) and microemulsions (Sierra, M. L. et al., J. Colloid Interface Sci. 212 (1):162-166 (1999)) and reported no effect in either case. However, this is hardly surprising because the amount of dissolved gas present in the aqueous phase, at the mM level, could not possibly be sufficient to influence these high surface area systems. Recently, a study of the coagulation rates of solid paraffin wax mixed with carboxylic acid stabilizer has shown that partial degassing (to about 97% using a water aspirator) has a significant effect on reducing the initial coagulation rates (Alfridsson, M. et al., Langmuir 16(26):10087-10091 (2000)). More recently, Ishida et al. (Ishida, N. et al., Langmuir 16(13):5681-5687 (2000)) have reported the complete absence of a long-range attraction (beyond 10 nm) from force measurements on systems that have never been exposed to dissolved gases.
Unfortunately, the literature produced over the last 20 years contains many reports of force measurements between hydrophobic surfaces with widely differing results, often depending on the preparation of the surfaces and the method used. There is therefore a need to better understand the electrostatic potential on an oil droplet or a hydrophobic particle and whether this potential is sufficient to stabilize fine droplets and to thus enable the formation of stable colloidal suspensions.